It is well known the for optimum power transfer from a RFPS to a load an impedance-matching circuit is required. The output power of a CO2 gas discharge laser increases directly with increasing discharge volume. The RF input (load) impedance of the laser varies inversely as the output power and decreases directly as the area of the discharge. The load impedance can vary between lasers within the same model family due to variations in discharge gas pressure, spacing between the electrodes, and other factors. Because of this, it is desirable to have an impedance-matching circuit that has a selectively variable impedance, such that a power supply having a fixed output impedance can be matched precisely to any one of a family of lasers having a load impedance anywhere within an anticipated range for that family.
U.S. patent application Ser. No. 12/786,199, by Hauer et al., assigned to the assignee of the present invention, and the complete disclosure of which is hereby incorporated by reference, describes an impedance-matching circuit comprising two series-connected lengths of transmission line, each thereof having an electrical length of about one-twelfth ( 1/12) wavelength at an operating frequency of the circuit, for matching the impedance of an RF power supply to a load. The impedance of the circuit is made selectively variable by connecting a variable impedance shunt to ground to a node between the two transmission line lengths. The variable impedance shunt can be a variable resistor, a variable capacitor, or a variable inductor. This arrangement has an advantage that a range of load impedances can be accommodated with a common transmission-line length. This reduces parts inventory in a manufacturing process for a family of lasers.
A disadvantage of the above discussed Hauer et al. arrangement is that it provides only for matching the impedance of a single-ended power supply to a load. In driving high power lasers it may be desirable to use in the power supply the output of a plurality of RF amplifiers with each being a separate output (end) of the power supply. This is because the power output of one RF amplifier can be limited by the power handling capability of amplifier components such as power transistors. There is a need for an impedance-matching circuit that is capable of connecting a multi-ended power supply to a load while still preserving advantages of the Hauer et al. arrangement.